Tutorial creates confusion about slices

[Michael Bentley]

On Apr 24, 2007, at 1:39 AM, Antoon Pardon wrote:

> On 2007-04-23, Michael Bentley <michael at jedimindworks.com> wrote:
>>
>> On Apr 23, 2007, at 7:38 AM, Antoon Pardon wrote:
>>
>>> The following is part of the explanation on slices in the
>>> tutorial:
>>>
>>> The best way to remember how slices work is to think of the  
>>> indices as
>>> pointing between characters, with the left edge of the first  
>>> character
>>> numbered 0. Then the right edge of the last character of a string  
>>> of n
>>> characters has index n, for example:
>>>
>>>   +---+---+---+---+---+
>>>   | H | e | l | p | A |
>>>   +---+---+---+---+---+
>>>   0   1   2   3   4   5
>>>  -5  -4  -3  -2  -1
>>>
>>> This is all very well with a simple slice like:
>>>
>>>   "HelpA"[2:4]    =>     "lp"
>>>
>>>
>>> But it give the wrong idea when using the following extended slice:
>>>
>>>   "HelpA"[4:2:-1]   =>   "Ap"
>>>
>>> So this doesn't result in the reverse of the previous expression  
>>> while
>>> the explanation above suggest it does.
>>>
>>>
>>> So I suggest to drop this.
>>
>> But 'drop' means to let or make (something) fall vertically...  :-)
>>
>> At that point in the tutorial, step values had not been discussed.
>> Just a bit lower down on the page you'll find a link to 'Sequence
>> Types' where you'll find an explanation of stepping you'll perhaps
>> find more satisfactory.
>
> That is very well posible. The question: Even if we get a good
> explanation later, do we want an explanation here that can cause
> confusion. These things are not just read and then discarded.
> Someone can already have read the whole tutorial and then come
> back to this place. So at that point he knows about stepping
> when he is reading this.
>
>
> I suspect that if you give this explanation to someone and explain
> that there is also a step parameter, chances are he will answer
> correctly if you ask him, what he thinks the following will result
> in:
>
>
>   "This is an example line"[12:19:2]
>
>
>
> If you ask him what the following will result in:
>
>   "This is an example line"[19:12:-1]
>
> Chances are he will give the wrong answer.

To be honest, bro -- I'd expect him to have enough intelligence to  
experiment for a second and figure it out.  This isn't rocket science  
-- you can plainly see what's happening -- so learn it and move on.   
Or better yet, quietly submit a patch...